1. Field of the Invention
The present invention relates to a lithographic apparatus, an article support, and a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In lithographic processing, passing of the beam through gas compositions present between the illumination system and the articles to be illuminated, in particular non-homogenous gas compositions, may cause undesired effects such as absorption, diffraction and refraction. These effects may have an adverse effect on illumination quality, in particular on a resolution to be reached for the ever increasing demand in imaging performance. A new generation of lithography, the EUV-lithography, which uses a beam in the Extreme Ultraviolet area, therefore operates in or near vacuum conditions in order to allow to pass the beam of radiation substantially unhindered to an article to be placed in the beam.
This vacuum technology offers challenges in terms of temperature control. For the article support, only a very small part (ranging from 0.1 to 3% of a total area) of the bottom side of the article actually mates physical contact with the article support when supported thereby, since the protrusions are shaped to provide only a very small contact area and the protrusions are furthermore arranged spaced relatively wide apart. In the vacuum pressure ranges that are used, thermal conductivity is substantially proportional to the pressure, which means that the thermal energy absorbed by the article when placed in the beam can no longer adequately be diverted, so that unwanted thermal heating of the article supports, leading to thermal expansion and resulting projection inaccuracies or potentially to even the loss of the article. To overcome this problem, a back-fill gas is introduced on the backside of the article offering a thermal conduction from the article to the article support to divert the thermal energy absorbed by the article. An electrostatic wafer support is for instance known from U.S. Pat. No. 6,628,503.
In the context of this application, the “article” may be any of the above mentioned terms wafer, reticle, mask, or substrate, more specifically terms such as: a substrate to be processed in manufacturing devices employing lithographic projection techniques; or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
For a proper and quick proliferation of the backfill gas along the supporting area of the article support, a channel is present that provides channeling of the backfill gas over the entire support area in a quick manner. The channel usually comprises a linked pattern of trenches or troughs, that substantially transports the backfill gas to the entire area of the support.
Conventionally, the article support is provided with protrusions that are arranged to improve the flatness of the substrate. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other and thereby form a bed of supporting members that support the substrate. Typically, the height of the protrusions lies in the range 1 mu m–15 mu m. Due to the relative large spaces in between the protrusions, contaminations possibly present generally do not form an obstruction for the flatness of the substrate, since these will be lying in between the protrusions and will not lift the substrate locally. The gap that is formed by these protrusions between the dielectric and the backside of the article is small enough to prevent the occurrence of breakdown, once electrode is energized and the article is clamped. However, it has been found, that in the channel, the local gap defined by the height between the bottom of the channel and the backside of the article increases substantially relative to the normal gap height between the dielectric and the backside of the article. This provides a substantial risk of voltage breakthrough in the gas feed region. In addition, there is a desire to design the channel in such a way that the clamp may be used in non-vacuum conditions, since for tuning and testing purposes, it is convenient to operate the electrostatic clamp without applying vacuum. Especially in non-vacuum conditions, breakthrough is likely to occur when the clamp is turned on.